The purpose of this Mentored Research Scientist Development Award (K01) is threefold. This award will allow me to: 1) learn a new skill set (social network analysis; SNA), 2) broaden my understanding of the social epidemiology of HIV among drug users in a new cultural and environmental context (the US/Mexico border), and 3) facilitate my transition to becoming a productive NIH-funded independent investigator at the University of California San Diego. The training aims will be accomplished through a combination of specific workshops and coursework, a hands-on practicum, and one-on-one mentoring with a Training Committee comprised of experts in the areas of HIV prevention research among vulnerable populations, behavioral intervention design, social network analysis, and HIV social epidemiology research in international contexts (and specifically the US/Mexico border region). The research aims will be accomplished by conducting a cross-sectional study of the social network factors associated with HIV risk behavior among 200 drug-using male clients of female sex workers (FSWs) in Tijuana, Mexico. The US/Mexico border is experiencing a burgeoning HIV epidemic, concentrated among high-risk groups such as drug users and FSWs. Social network analysis (SNA) has been used for many years in the US to understand the role of drug user social networks in regional HIV transmission, but rarely has it been used between regions or across international borders, which are characterized by high levels of cross-border mobility. Understanding the role of social networks in highly mobile bridging populations is critical to a more nuanced and contextualized understanding of infectious disease transmission in border regions. The proposed research will allow me to apply newly acquired skills in SNA to evaluate the structure, composition, and influence of social networks, and the degree of interaction among drug, sex, and social networks of a particularly high-risk group: US and Tijuana-based drug-using male clients of FSWs. This population exhibits high rates of drug use (25% report ever injecting drugs, and up to 88% report lifetime use of cocaine, heroin, or methamphetamine) and considerable sexual risk behavior with both FSWs and other partners (including 66% who report concurrent sex and drug use). Findings from the proposed research will be critical for developing tailored interventions for individuals at risk for HIV via sex and drug use in border regions, and for the development of future research into the mobility-HIV risk pathway. Further, developing skills in social network analysis will uniquely position me as one of only a handful of mixed-methods researchers in the field of drug abuse possessing such skills, and the only one in the Division of Global Public Health at the University of California San Diego.